The use of biomass and biomass-derived feedstocks in the production of fine and commodity chemicals is hindered by the lack of cost effective purification and conversion processes. The development of efficient and selective catalytic transformations should allow biomass feedstocks to become economically competitive with traditional petroleum-based feedstocks. Ideally, the conversion reactions will be applicable to a range of substrates and tolerant of crude biomass inputs.
Glycerol is of particular interest as a renewable raw material, because it is currently being generated in vast quantities as a byproduct of biodiesel production. The attractiveness of glycerol as a sustainable platform chemical has inspired research efforts to use the triol as a building block for a variety of functionalized and reduced structures.
The partial hydrogenolysis of glycerol has been envisioned as a new, direct route to 1,3-propanediol (1,3-PD) for the production of polyesters, polyurethanes, and polyethers with high renewable resource content. Particularly interesting is the production of polytrimethylene ether glycol from 1,3-PD. One strategy for selective reduction of glycerol is a tandem catalytic sequence involving the dehydration of secondary alcohols followed by hydrogenation of the resulting carbonyl groups. Several competing condensation and dehydration pathways are available to glycerol in acidic aqueous medium, which typically leads to complex product profiles and low selectivity.
Efficient conversion of glycerol to higher value products will require the development of highly active, selective catalysts. It will be important to identify robust catalysts that are tolerant of water and acid at the high temperatures required for the dehydration step. For example, biodiesel production employs base catalyzed transesterification of triglycerides with methanol, which generates approximately 10% byproduct glycerol. This has flooded the glycerol market. The price of crude glycerol has dropped dramatically. Biodiesel production is expected to increase significantly in the next decade. One solution to the “glycerol glut” problem is to catalytically convert glycerol to a higher value product. Targets can include 1,3-propanediol, 1-propanol, and 2-propanol. Of the possible products, 1,3-propanediol is most valuable, having a ready application in the production of polyesters. For example, Dupont markets this polyester as Sorona®; the current commercial process for 1,3-propanediol employs fermentation of glucose using genetically engineered microorganisms. Previous reports of glycerol hydrogenolysis have employed both homogeneous and heterogeneous catalysts. Selectivity is generally poor, with a wide range of products including ethers, esters, alcohols and diols. Most reported procedures have employed organic solvents.
Early attempts to demonstrate the feasibility of selective conversion of glycerol to 1,3-PD utilized homogeneous catalysts yielding at best 21% 1,3-PD with 45% selectivity using a Rh(CO)2(acac) complex with tungstic acid (H2WO4) at 200° C. under 313 atm of H2/CO. Early work with heterogeneous catalysts such as copper chromite showed selectivity for 1,2-PD, with <5% 1,3-PD. More recently, heterogeneous catalysts have been identified that show improved selectivity for 1,3-PD. With Pt on sulfonated ZrO2 selectivities of up to 56% for 1,3-PD were reported.
Homogenous catalysts have a distinct advantage in that the application of mechanistic understanding can allow rational tuning of catalyst structure and reaction conditions to optimize activity, selectivity and lifetimes. Schlaf and coworkers have explored the reduction of terminal vicinal diols to n-alcohols using homogeneous Ru ionic hydrogenation catalysts and through these studies have provided insight into various factors that control deoxygenation selectivity. The use of diol models, which are less reactive than their polyol analogs, enabled kinetic analysis and more thorough characterization of product profiles. The most selective of the Ru hydrogenation catalysts was [Cp*Ru(CO)2(H2)][OTf], which achieved a 54% yield of 1-propanol in sulfolane solvent at 110° C. and 710 psi H2 using trifluoromethanesulfonic acid (HOTf) as the catalyst for the initial dehydration step. In addition to the 1-propanol hydrogenation product, the formation of several ether condensation products was observed, including significant yields of propylene glycol propyl ether (11%) and di-n-propyl ether (15%). The overall selectivity for reduction of the secondary hydroxyl group is 99%. The high regioselectivity of the ruthenium catalyst was impressive, but the catalyst is deactivated by water, which is a byproduct of the reaction. The active Ru dihydrogen complex is deprotonated by water to yield an inactive dimer. This catalyst decomposition makes it impractical for use with inherently wet glycerol from biodiesel production. Several ruthenium containing ionic hydrogenation catalysts with N-donor ligands have demonstrated greater solubility and stability in aqueous solution, however significant thermal decomposition and/or reduced selectivities were observed.
The homogeneous ruthenium systems reported to date represent significant advances in the development of selective polyol deoxygenation catalysts, demonstrating unprecedented regioselectivity in partial diol deoxygenation. In addition, studies of these systems have delineated many of the competitive equilibria and condensation pathways that are common to alcohols in acidic medium and must be minimized for effective conversion. As vicinal diol groups are common to all sugar polyols, the lessons learned using these model substrates are expected to be broadly applicable. To fully develop and optimize effective polyol deoxygenation technologies, catalysts that exhibit greater stability at high temperatures in the presence of water are needed.
Thus, there is a need for catalysts and processes that can efficiently convert polyols (e.g., glycerol) into other products, such as 1,3-propanediol as well as others.